1. Field of the Invention
The present invention relates to: an electrophotograph; an image forming method for visualizing an electrostatic image; and a magnetic toner for toner jetting.
2. Description of the Related Art
In recent years, an image forming apparatus has been further requested to have a high-speed process and long-term high reliability in addition to high definition, high appearance quality, and high image quality. A reduction of toner particle size and a sharpening of a toner particle size distribution have been attempted to achieve a high-resolution and high-definition development method. However, when the particle size of toner is merely reduced, dispersibility of a internal additive into a binder resin is apt to affect toner performance.
In particular, in the case of magnetic toner having magnetic toner particles used for a one-component development method in which a reduction in size of an image forming apparatus is advantageous, various properties requested for the magnetic toner such as development property and durability may be affected by the state of dispersion of magnetic material particles in the magnetic toner particles.
When magnetic material particles are insufficiently dispersed into magnetic toner particles, the total amount of magnetic material particles exposed to the magnetic toner particle surfaces is different from one another. When the amount of magnetic material particles on the magnetic toner particle surfaces is small, the magnetic toner particle surfaces have high charge amounts when they are subjected to triboelectric charging with a charge imparting member (developing sleeve), so charge-up occurs.
On the other hand, when the amount of magnetic material particles on the magnetic toner particle surface is excessively large, charge is apt to leak, so a high charge amount is hardly obtained. Moreover, a toner of opposite charging polarity is apt to generate owing to contact between any one of the magnetic material particles and a binder resin, so the width of a charge distribution expands. The expansion may be responsible for the deterioration of image quality.
Meanwhile, a contact charging method has been adopted in many cases, which involves charging a photosensitive member by means of a contact charging member without the use of a corona charging device that generates ozone. When magnetic material particles are not uniformly dispersed into magnetic toner particles, magnetic toner the surface of which has an excessive amount of magnetic material particles receives mechanical pressure or electrical compression at an abutting portion of the contact charging device and the surface of the photosensitive member, so the surfaces of both members are strongly rubbed with the toner. As a result, a flaw on the photosensitive member is apt to develop, and the flaw may be responsible for an image defect. In contrast, when both the contact charging member and the photosensitive member are strongly rubbed with magnetic toner the surface of which has a small amount of magnetic material particles and has an apparently increased viscoelasticity, the toner is apt to fuse to the photosensitive member. As a result, a contamination of the photosensitive member such as filming is apt to occur.
In general, an external additive is added to magnetic toner particles for improving the fluidity of magnetic toner. However, when the magnetic toner having the external additive deteriorates owing to repetition of a printing step over a long period of time, the external additive is embedded into the magnetic toner particles, so influences of magnetic material particles exposed to magnetic toner particles surfaces become great. As a result, such problems as described above are apt to be remarkable.
In addition, the expansion of the width of a charge distribution due to insufficient dispersion of magnetic material particles as described above is apt to cause so-called selective development in which toner having a certain range of charge amount distribution is selectively consumed. Moreover, the progress of the selective development is apt to further accelerate various problems.
For example, the charging property of magnetic toner becomes susceptible to the environment. In addition, the fluidity of the magnetic toner is reduced, so the toner is insufficiently supplied to a developing sleeve, and charge unevenness of a toner layer on the developing sleeve is caused. Accordingly, “fogging” in which a non-image portion is developed with the toner is apt to occur. In a high-temperature-and-high-humidity environment, a phenomenon called “fading” in which an image density is reduced in a belt fashion tend to occur.
Furthermore, when a toner is transferred from a photosensitive member onto a transfer material, if the toner is excessively charged, a phenomenon called “scattering” is caused, in which the toner is scattered around a letter image or a line image. If a toner has been insufficiently charged for the purpose of suppressing the scattering, the reduction of developability and the fogging may be caused. Additional sharpening of the toner particle size distribution has been attempted to suppress the fogging. However, the sharpening may be a factor for increasing toner production cost due to, for example, a reduction in yield.
In particular, in order to cope with recent trends such as an increasing process speed and an extending lifetime, a high-speed developing system has been employed or a large-capacity process cartridge amount of toner in which is increased has been used. However, these coping with the recent trends tend to make the above problems more remarkable, so quick alleviation of such state has been desired.
JP 03-101743 A and JP 03-101744 A each disclose that the particle sizes of magnetic material particles are reduced and a particle size distribution is narrowed for uniformly dispersing the magnetic material particles into magnetic toner particles. Those measures surely tend to uniformize the dispersion of the magnetic material particles into the magnetic toner particles. However, when the particle size of magnetic toner is reduced for achieving high image quality, the fogging is accelerated. Therefore, the dispersibility of magnetic material particles into magnetic toner particles is still susceptible to improvement.
There also arises a problem of declining degree of blackness in the case that the particle sizes of magnetic material particles are reduced. It has been conventionally known that the degree of blackness of magnetic material particles depends on the content of FeO (or Fe2+). However, the FeO (or Fe2+) content in the magnetic material particles reduces as deterioration with time due to oxidation proceeds, with the result that the degree of blackness of the magnetic material particles reduces. It is needless to say that the deterioration with time largely depends on the environment where the magnetic material particles are placed. The deterioration is also accelerated by the reduction of particle sizes of the magnetic material particles.
Magnetic material particles with reduced particle sizes are susceptible to heat as well as time. In order to uniformly disperse magnetic material particles into magnetic toner particles in the production step of magnetic toner, it is preferable that a melting and kneading temperature is set at a high temperature and a binder resin is kneaded after it has been melted to be soft. In particular, when a binder resin containing a hard component such as THF insoluble matter is used, the binder resin is preferably softened and kneaded at a high temperature in order to uniformly disperse magnetic material particles into the binder resin. Even magnetic material particles having high degree of blackness can be oxidized depending on the particle sizes of the magnetic material particles and on the melting and kneading temperature in the toner production process, with the result that toner that looks reddish may be finally obtained.
In general, a polyester resin is preferably used as a binder resin than a styrene-based resin from the view of obtaining toner excellent in low-temperature fixability. However, the polyester resin has a large number of acidic functional groups in its molecular structure, so magnetic material particles in the polyester resin are placed in an acidic environment in the kneading process. Accordingly, the oxidization of magnetic material particles in the polyester resin tends to proceed in particular.
To solve those problems, a large number of proposals have been conventionally made, in each of which various elements are added to magnetic material particles in magnetic toner particles.
JP 08-133744 A and JP 08-133745 A each disclose a magnetic material particle coated with a coating layer containing an element selected from the group consisting of Si, Al, and Ti.
However, these magnetic material particles may cause defects concerning development. For example, these magnetic material particles for uniformly being dispersed into magnetic toner particles are insufficient for the prevention of a reduction of blackness and an improvement of heat resistance. Furthermore, the magnetic material particles can be oxidized when kneaded at a high temperature. And the magnetic properties of the magnetic material particles can be affected by some of the elements to be added. In particular, when the magnetic material particles are used in combination with a resin having a relatively high acid value, the added elements are apt to be eluted from the magnetic material particles.
Magnetic material particles have also been known, each of which contains 1.7 to 4.5 atom % of Si based on Fe atom, and 0 to 10 atom % of one or two or more metal elements selected from the group consisting of Mn, Zn, Ni, Cu, Al, and Ti based on Fe atom (see, for example, JP 09-59024 A1 and JP 09-59025 A1). The magnetic material particles can improve the magnetic properties and charging property of magnetic toner. However, when the above metals are merely added to the magnetic material particles, compatibility between a developability of the magnetic toner and an image quality and the like in a high-speed developing system are still susceptible to improvement.
Such magnetic material particles as described below have also been known (see, for example, JP 11-157843 A). Each of the magnetic material particles includes Si component in its particle center to particle surface continuously. And a part of the Si component is exposed to the magnetic material particle surface. In addition, the outer shell of each of the magnetic material particles is coated with a metal compound comprising at least one metal component selected from the group consisting of Zn, Mn, Cu, Ni, Co, Cr, Cd, Al, Sn, Mg, and Ti. The metal compound is bound to the Si component. The use of such magnetic material particles shows good developability at an initial stage of printing duration, however could not alleviate a reduction of image quality or developability such as the acceleration of fogging due to long-term use, particularly in a high-speed developing system. Therefore the magnetic material particles are still susceptible to improvement.
Such magnetic material particles as described below have also been known (see, for example, JP 11-189420 A). Each of the magnetic material particles contains Si component and Al component. Those components are presented in its center to its surface continuously, and a part pf those components are exposed to the particle surface. In addition, the outer shell of each of the particles is coated with a metal compound comprising at least one metal component selected from the group consisting of Zn, Mn, Cu, Ni, Co, Cr, Cd, Al, Sn, Mg, and Ti. The metal compound is bound to the Si component and the Al component. However, the use of such magnetic material particles could not imparted sufficient charging stability to magnetic toner yet.
JP 07-239571 A discloses a magnetic material particle having on its surface an oxide containing an element selected from the group consisting of iron, aluminum, titanium, zirconium, and silicon. JP 07-267646 A discloses a magnetic material particle having an element selected from the group consisting of Zn, Mn, Cu, Ni, Co, Mg, Cd, Al, Cr, V, Mo, Ti, and Sn. JP 10-72218 A discloses a magnetic material particle having on its surface an element selected from the group consisting of Si, Al, Ti, Zr, Mn, Mg, and Zn.
JP 07-240306 A discloses such a spherical magnetic material particle described below. The particle contains 0.10 to 1.00 mass % of a silicon element in it. A coprecipitate of silica and alumina is present on the surface of the particle. Furthermore, at least one kind of fine particle powder of non-magnetic oxide or non-magnetic water-containing oxide of element selected from the group consisting of Fe, Ti, Zr, Si, and Al is fixed in an amount of 0.1 to 10 wt % to the coprecipitate. JP 10-171157 A discloses a hexahedral magnetic material particle containing 0.9 atom % or more and less than 1.7 atom % of Si based on Fe atom and having on its particle surface an adherend layer composed of an oxide, hydroxide, water-containing oxide, or a mixture thereof of one or two or more kinds of elements selected from the group consisting of Mn, Zn, Ti, Zr, Si, and Al.
JP 2003-195560 A and JP 2004-139071 A disclose a toner containing a magnetic material particle having an isoelectric point of 5 to 6.5 and a magnetic material particle having an isoelectric point of 5 to 9, respectively.
Furthermore, JP 2004-78055 A discloses a toner with its amount of adsorbed moisture and average circularity specified.
In each of JP 08-34617 A, JP 03-2276 A, JP 2003-192352 A1, and JP 2003-162089 A1, a titanium compound is allowed to be in a magnetic material particle or on the surface thereof.
A good developability may be achieved by each of the above inventions. However, when each of them is applied to a high-speed developing system having a high process speed and employing a large-capacity cartridge, additional improvements of developability and durability are desired in many cases. Furthermore, when the amount of an added element is excessively large or the smoothness of the surface of a magnetic material particle is not good, the amount of moisture adsorbed to the magnetic material particles becomes excessively large. As a result, various problems arise from the fluidity or chargeability of magnetic toner such as a flaw or filming on a photosensitive member, fogging or fading of image, or scattering of the toner. In addition, resistance to oxidation and stability of the magnetic material particles are still susceptible to improvement, in the case that the magnetic material particles is mixed with a resin having a relatively high acid value such as a polyester resin in the production process of toner particles.
In addition, depending on the kind and amount of added element, the amount of moisture adsorbed to a magnetic material particle becomes excessively large or the adsorbed moisture is hardly desorbed. As a result, problems are apt to occur such as a significant reduction in chargeability of the toner after standing particularly in a high-temperature-and-high-humidity environment.
As described above, the realization of magnetic toner which is excellent in durability and developability even when it is applied to a high-speed developing system having a high process speed and employing a large-capacity cartridge, and which has high degree of blackness requires further investigation.